Piggy is important because of what he represents. Piggy is the only character that doesn't get pulled down into the base evil that the other characters delve in to. Even Ralph expresses enjoyment at killing a pig and participates in the dance that frenzies the boys so much that Simon is killed.

Piggy represents an adult, civilized world. A world that is ruled by law and order and that punishes violent acts against other people. Piggy understands the need for a leader, and even though he didn't vote for Ralph, Piggy always shows his support to Ralph because Ralph was elected chief.

Piggy is also an important character because of his education and intelligence. Piggy not only knows that a fire is important, but also how his glasses can be used to start the fire. That knowledge makes him important to the story AND to the boys stranded on the island.